        .text
        .globl main
main:
        addi $s1, $zero, 20
        addi $s2, $zero, 2000
        addi $s3, $zero, 42
        addi $s4, $zero, 81

        add $t0, $s1, $s2
        add $t1, $s3, $s4
        sub $s1, $t1, $t0
        jr $ra          # retrun to caller
